Sauté the base: Heat olive oil in a large pot over medium heat. Add onion and cook until soft (about 10 minutes). Add garlic and carrots and cook for another 10 minutes.
Add spices: Stir in cumin, smoked paprika, turmeric, and thyme. Let them toast for 1 minute.
Combine the main ingredients: Add diced tomatoes (with juice), lentils, and diced potato. Stir to combine.
Simmer: Pour in vegetable broth. Bring to a boil, then reduce heat and simmer uncovered for 35–45 minutes, or until lentils and potatoes are tender.
Finish and season: Stir in lemon juice and chopped cilantro. Season with salt and pepper to taste.